The hip-joint is a massive ball and socket joint formed by the rounded head of the femur and the cup-shaped socket of the acetabulum. The articulating surfaces are covered with cartilage and held together by bands of ligaments and overlying muscle. The hip-joint is capable of several types of movement, consisting of flexion, extension, adduction, abduction, circumduction and rotation.
When the hip-joint is subjected to surgery, the complex layered structure of cartilage, ligament and muscle is necessarily cut and otherwise disturbed, leaving the joint substantially immobile to begin the healing process. Within a relatively short time after surgery, it is desirable to begin movement of the patient's leg about the hip-joint and to increase both the extent and duration of such movement progressively. Since the patient is usually unable for the first several days to stand on the leg having the hip-joint surgery, a need has arisen for a passive exercise device to restore the hip-joint and related muscles to full utility.